At first you have to set priority. For example in atm machine, its processing and response time must be less as require by any atm machine users. After that create test cases on the basis of scenarios. All scenarios should be included in your test cases.

Yes processing and response time is important. Some points/scenarios to keep in mind when writing ATM testcases are...1. The denominations of the cash dispensed by ATM. The 100s, 500s or 1000s notes dispensed by ATM would depend on the amount requested by user. Like for smaller amounts 100s are enough, for medium amounts 500s and 100s and for larger amount we can have a combination of 1000s, 500s and 100s2. The receipt generated should correctly reflect the amount withdrawn and the balance3. If ATM machine provides other services like changing pin number, updating customer bank records, paying bills etc4. The instructions and graphics should be clear. we should use arrows for service selection if buttons are present OR if it is touch screen then it must be clearly mentioned where customer can tap on the screen5. The card pin and account validation